In the Bayer process for the extraction of alumina from alumina containing ores, precipitated alumina tri-hydrate is filtered, dried and calcined. This yields high purity alumina with a narrow range of particle sizes. However, a by-product of the calcination process is extremely fine particles of alumina, with an average size of less than 30 μm. This by-product is caught in the gas cleaning devices, including multi-cyclones, electrostatic precipitators and/or bag-houses, attached to the calcination device. Such particles are commonly known as ESP dust. In addition to being difficult to handle, ESP dust is difficult to reintroduce into the process stream, not being readily redigested in the highly caustic solution of the digestion phase.
Accordingly, it is desirable to be able to agglomerate small particles of alumina, such as ESP dust, to form coarser particles. Ideally, these coarser particles are of a size range suitable for use in aluminium smelting.
In the ceramics industry, particles are agglomerated by spray drying using organic polymers as binders. However, the agglomerates formed by these methods are typically weakly bonded and they are readily degraded when handled or transported.
In Australian Patent 664328, there is provided a method for agglomerating alumina particles with a binder comprising a polymer form of a hydroxy salt of aluminium. In a variant of the invention, activated alumina is used to reduce the quantities of the binding agent required. The activated alumina is believed to enhance binding by forming a film of pseudo boehmite that adheres to the particles of unactivated alumina powder. However, it is clearly stated that the activated alumina cannot be used alone to achieve beneficial agglomeration of the alumina particles.
However the cost of the binding agent, and that associated with the production of activated alumina, have prevented the widespread application of this method. It is one object of the present invention to provide a method for the agglomeration of particles of alumina, or particles comprising alumina, where the quantity of binder required is substantially reduced, and the need for activated alumina eliminated.
The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.
Throughout the specification, unless the context requires otherwise, the word “alumina” will be understood to encompass fully dehydrated alumina, fully hydrated alumina, partially hydrated alumina or a mixture of these forms.
Throughout the specification, unless the context requires otherwise, the term “alumina particles” will be understood to include particles of an alumina containing material where the alumina content of said particles is at least about 30% by weight Al2O3.
Further, throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.